BIVALVIA. 47 



Arca Simula, Phil. Tab. V, fig. 17. 



Aeca jemula, Phil. Geol. Yorksh., i, t. 3, f. 29, 1835. 



Testa subrhomboided, vel oblongd, inaquilaterali, convexd, umbonibus obliquis anteme- 

 dianis distantibus ; latere antico convexo, latere postico elotigato, obtusi carinato et 

 compresso ; superficie sulco lato mediano ; basi subrectd ; lineis radiantibus crebris minutis 

 nodosis, plicis concentricis paucis interrnptis : superficie posticd excavatd, plico unico 

 obliquo mediano et sulcis duobus conformibus parallelis. 



Shell subrhomboidal or oblong, inequilateral, convex, umbones oblique, placed anterior 

 to the middle of the valves and separated by a moderately wide area ; anterior side convex, 

 posterior side elongated, obtusely carinated and compressed; the dorsal surface with a 

 wide and slightly oblique depression which is not always distinct ; basal margin straight ; 

 radiating lines closely arranged, fine, minute, and knotted, interrupted by a few concentric 

 plications ; the surface posterior to the obtuse carina is concave, has a mesial oblique 

 plication which is bounded upon each side by a sulcation. 



The greater number of examples have not preserved the lines which ornament the 

 surface, but the posterior plications are always distinctly shown. In the Great Oolite of 

 Minchinhampton, the species occurs in a dwarfed or rather in an immature form, which 

 would scarcely be identified, but for the aid of specimens from other localities ; it occurs 

 well preserved in the shelly roe stone of Leckhampton Hill, and likewise in the Ponton 

 Oohte, at both of which places it attains its full dimensions. 



Localities. Minchinhampton and Bisley Commons. Ponton, Lincolnshire. 



Arca mt&vlk, Phil. var. transversa. Tab. V, fig. 8. 



A shell which we consider to be only a variety of A. amulet requires a separate notice, 

 it is more elongated and subcylindrical, the mesial or oblique depression upon the dorsal 

 surface is usually distinctly marked, and the shell never acquires the dimensions of the 

 typical form, the length of the largest specimens not exceeding 10 lines. 



The more mesial position of the umbones will serve to distinguish this shell from young 

 examples of Macrodon Hirsonensis, to which in other respects it has a considerable resem- 

 blance ; it is more elongated and cylindrical than any other of the contemporaneous Arcacea. 



It occurs not uncommonly throughout the shelly beds of the Great Oolite, but the 

 delicate features of its surface are seldom well preserved. 



Localities. Minchinhampton and Bisley Commons. 



Arca rtjgosa? var. of Arca Prattii. Tab. V, fig. 2. 



Testa subrhomboided, convexd, antice rotunda, postice compressd, angulo obliquo acuto ; 

 umbonibus depressis approximates, antemedianis lineis radiantibus crebris undulatis subno- 

 dosis et imbricatis ; plicis concentricis ruijis, irrer/idaribus subundulatis ; basi subsinuatd. 



Shell subrhomboidal, convex, anterior side rounded, posterior side much compressed, 



